There is an imaging device in which a living body is illuminated with electromagnetic waves, such as near-infrared rays, and then an image of the electromagnetic waves that have passed through the living body or have been diffusely reflected by the living body is captured, so that internal body information (such as blood vessel patterns of the living body) is acquired. The imaging device is mainly used for personal authentication systems that use biometric information.
There are technologies for reducing blurs caused by scattering caused by the living body in an imaging device that acquires internal body information. For example, there is a technology in which electromagnetic waves are caused to pass through a living body, an image of the electromagnetic waves that have passed through the living body is captured, and image reconstruction processing is applied to the captured image, so that the influence of scattering caused by the living body is reduced and thus a captured image of the internals of a body is obtained (see “Near-infrared imaging in vivo: imaging of Hb oxygenation in living tissues”, SPIE Vol. 1431 Time-Resolved Spectroscopy and Imaging of Tissues (1991) pages 321-332, Ryuichiro Araki and Ichiro Nashimoto, for example). The technology is a transmission-type technology.
Unfortunately, in the related art technologies, it has proved difficult to capture a clearer image of the inside of a photographic subject that exhibits a light scattering property. Photographic subjects exhibiting a light scattering property include a living body that scatters infrared light.
The reason why it is difficult to capture a clear image is that light is scattered when passing through the inside of a photographic subject and therefore it is difficult to obtain information on portions of the inside located at a depth equal to or greater than a given level, as an image, and spatial information is lost. Here, diffuse reflection will be described.
Some of the light applied to a photographic subject is reflected by the surface of the photographic subject in accordance with Fresnel's formula. Fresnel reflection is reflection that occurs because of the difference in the refractive index between the photographic subject and the medium (such as air). The remaining light for which Fresnel reflection has not occurred penetrates inside the living body.
Inside a photographic subject exhibiting a light scattering property (a body exhibiting a light scattering property is hereinafter also referred to as a “scatterer”), light is not able to travel linearly, and travels while continuously changing travelling direction. FIG. 1 illustrates an example simulation result of propagation of light inside of a living body.
The example illustrated in FIG. 1 illustrates the manner in which one photon that has entered a living body travels. FIG. 2 is a table listing the simulation conditions of the simulation illustrated in FIG. 1. As illustrated in FIG. 2, the refractive index, the absorption index, the scattering coefficient, and the anisotropic parameter are set for both the air layer and the living body layer. Each condition is described below.
(1) Refractive index n: the refractive index is a value obtained by dividing the velocity of light in a vacuum by the velocity of light in matter (namely, phase velocity), and is an index used to describe how light travels through matter.
(2) Absorption coefficient μa: the absorption coefficient is a constant representing to what extent a medium absorbs light when the light enters the medium. Given that the intensity of light at the time of entrance into a medium is I0, and the intensity of light when light travels a distance x is I (x), the following expression (1) holds in accordance with Lambert-Beer's law.I(x)=I0e−μαx  (1)
When light travels inside a medium, the intensity of light decreases exponentially with respect to the distance traveled. The coefficient for the exponential decrease is absorption-index μa.
(3) Scattering coefficient μs: the scattering coefficient is a coefficient indicating the proportion of scattering when light propagates through a medium. Scattering may be expressed with the same formula as with the above absorption coefficient. Given that μs is a scattering coefficient, the intensity I of light that has traveled straight in a medium without being scattered may be represented by the following expression (2).I(x)=I0e−μsx  (2)
When light travels inside a medium, the intensity of light decreases exponentially because of scattering. The coefficient for the exponential decrease is the scattering coefficient μ.
(4) Anisotropic parameter g: the above scattering coefficient (that is, μs) indicates the frequency (probability) at which scattering occurs. In contrast, an anisotropic parameter g is a parameter indicating a direction to which the direction in which light travels is changed by scattering. FIG. 3 illustrates the scattering direction θ of light. The anisotropic parameter g is the average of cos θ given that the scattering angle is θ. The average of cos θ is denoted by <cos θ>.g=<cos θ>  (3)
<cos θ>: average of cos θ
FIG. 1 illustrates the result of a simulation of the motion of a photon under the above three conditions, that is, under the conditions that the probability of absorption is determined by μa, the probability of scattering is determined by μs, and the direction of scattering is determined by g.
As illustrated in FIG. 1, an image that is a signal acquired as the result of scattering of light inside the scatterer is in a blurred state. Accordingly, in related art technologies, it has proved difficult to obtain a clear image acquired at a depth equal to or greater than some value (2 to 3 mm) in the scatterer.
One technology using a captured image inside a photographic subject is palm vein authentication. For example, in a common imaging scheme used in palm vein authentication, it is possible to capture an image of veins at a depth of several millimeters (for example, 2 to 3 mm) below the skin. The common imaging scheme utilizes a near-infrared diffusion light source.
However, if the image of a blood vessel at a location that is deeper (for example, 3 to 7 mm) from the surface is able to be captured, authentication accuracy might significantly improve. This is because the amount of information used for authentication increases.
If information about a deep location under the skin is able to be clearly visualized, it may be possible to utilize the visualization for an injection assisting device that displays the locations of blood vessels for injection assistance, or detection of other pathological changes. Moreover, with regard to food such as meat, capturing an image of information on a deep location may enable visualization to be utilized for foreign substance inspection in food.
Accordingly, it is desirable that information on a location deeper than a depth captured by related art technology be able to be acquired as an image, for a photographic subject exhibiting a light scattering property.